The invention relates to a method for the metallic coverage of surfaces of high temperature superconductors (HTS) based on a Copper-Oxygen structure. High temperature superconductors because of their superior technical properties are used as electrical devices in the energy technique. The superconductors are of the chemical compositions Bi2Sr2CaCu2O8−x (Bi 2212) or Bi2Sr2Ca2Cu3O10+y (Bi2223) and the 123 family Y1Ba2Cu3O7-8 (Y123). The compounds for the technical application need electrical or thermal contacts, electrical or thermal shunts or surface layers for passivation and stabilization purpose. Although the Y123 superconductor exhibit excellent physical properties, the fabrication of large conductor length is to date unsucceeded and still a challenge. In contrast, Bi superconductors are produced in km length units. The Ag matrix is a prerequisite of the Bi wires, however this prevents some applications, e.g. the direct use for current leads and fault current limiters (FCL). The high electrical and thermal conductivity of Ag increases the desired low heat conductivity (current lead). The fast resistivity increase is necessary for the effective limiting behavior in case of an electrical fault. Again, the Ag matrix prevents the fast limiting behavior. A possible alternation are alloys of the type AgAu/Ag and Ag/AgMg which show in combination with the Bi HTS a lower electrical resistivity and a smaller coefficient of thermal conductivity compared to metalic silver. A disadvantage of this conductor type are the higher costs for the silver matrix.
German Patent DE 42 20 925 C2 discloses a powder metallic method for the fabrication of electrical contacts on HTS material whereby the contact are cold uniaxially pressed followed by a sinter procedure. The pressed powder consists either of HTS material or non-superconducting precursor are used. Furthermore, DE 44 18 050 A1 discloses a method for the inductive limiting of electrical faults using a superconducting hollow cylinder. Local overheating areas so called “hot spots” are prevented by metallic layers with a high specific resistivity >μΩcm2 at 77 K, especially by metallic conductors of lead, antimony, indium, bismuth, steel, tin or zinc including their alloys. The above patent application describes an HTS tube encircled with an arming metal shunt for the current limiting in the corresponding devices. The physical key of the shunt is the quality of the electrical contact between the metallic layer and the ceramic superconductor. Because most of the metals and alloys are not composition elements of the new superconductors (except bismuth) a strong bonding can hardly be generated. The metals tend to oxidize and the stability of the metal-HTS compound is influenced negatively. Long-time degradation will appear. The combination of bulk Bi2212 HTS with metallic conductors, preferred noble metals silver, gold, ruthenium, osmium rhodium, iridium, palladium or platinum is disclosed in DE 41 24 980 A1. In the s called melt cast process after DE 38 30 092 OS tubes f Bi2212 are fabricated. During the fabrication melting process metallic wires e.g. silver wires are included along the total tube length or a both ends. This wire stabilizes the superconductor and allows simultaneously a current lead function from the endgaps. This in-situ method is expensive and can destroy the homogeneous structure of the superconductor. Furthermore, at a temperature of 870° C. the process can produce different non-superconducting phases.
Accordingly, the key issue of the present invention relates to a method of metallic deposition on the surface of high temperature superconductors with a copper oxygen basic structure. A major advantage of the invention is the less expensive procedure by fabricating electrical contacts with a low electrical and thermal transfer resistance and with a high stability of the metallic layer. A principal solution is given in claim 1. Preferred embodiments of the invention are specified in the claims 2 to 10. In the further description the invention is explained by an example.